Our major objectives are to learn more about immunoglobulin gene expression in T cells, and to use our successful approach of fusing T cells to plasmacytomas in order to understand more about the regulation of immunoglobulin gene transcription. The following four studies were conducted. (1) Immunoglobulin gene transcription in T lymphoma-plasmacytoma hybrid cell lines has been highly successful. The transcriptional amplification of immunoglobulin gene expression which normally accompanies B-cell maturation does not occur in the above hybrids. This result suggests that transcriptional amplification requires translocated VH sequences. In contrast, the ratios of secreted:membrane-terminated Cmu RNAs increased dramatically in the hybrids. Hence, the regulation of muS:muM ratios is independent of amplification. In addition, we observed that muM fragments are present in cells producing high muS:muM ratios. The muM fragments now have been characterized in detail. These data provide strong evidence that production of muS termini does not require transcriptional termination 5' to the muM exons. (2) We demonstrated that transcription of Cmu RNAs in the T lymphoma ST4 is accompanied by DNA demethylation. This result is in accord with our conclusion that transcriptional activation of immunoglobulin genes does not require VH translocation. (3) The Cdelta gene is not transcribed in T lymphomas. Analysis of RNA from a variety of mu RNA-producing lymphomas revealed no trace of delta RNA. These data suggest that the Cdelta gene is not transcribed in such cells. (4) Extensive studies with a wide range of T lymphomas, T hybridomas, cloned T-cell lines, thymic and peripheral T-cell populations have revealed no evidence of VH gene expression. In our most sensitive experiments, one VH RNA molecule per 100 cells would have been detected. This data, and similar data now emerging from other laboratories, must cast serious doubt on the widely held belief that VH sequences constitute a component of the T-cell antigen receptor.